The present invention relates to a thermal head apparatus used in a thermal printer and, more particularly, to a compact, low-cost thermal head apparatus.
A thermal printer has a simple mechanism, and a thermal printer having a large number of heat generation elements serving as recording elements can be easily manufactured. For this reason, the thermal printer is popularly used. In the thermal printer, a thermal head apparatus constituted by heat generation elements and a driving circuit therefor is arranged.
FIG. 5 shows a conventional thermal head apparatus. This thermal head apparatus is constituted by 64 heat generation elements R1 to R64 and a heat generation driving integrated circuit 80, and the heat generation elements R1 to R64 and the heat generation driving integrated circuit 80 are formed on a single thermal head base 81. The heat generation driving integrated circuit 80 is constituted by a shift register unit 801, a latch unit 802, an output gate unit 803, and 64 output transistors Q1 to Q64, and each of the shift register unit 801, the latch unit 802, and the output gate unit 803 has output terminals corresponding to 64 bits. The heat generation driving integrated circuit 80 is arranged as described above to reduce the number of wiring lines between the thermal head apparatus and an external control circuit. In addition, the thermal head base 81 is constituted by, e.g., an alumina ceramic board or the like.
Print data is input as 1-line serial data (Serial-in) but not as 64-bit parallel data to the shift register unit 801 synchronized with a clock signal Clock. The print data is transmitted to the latch unit 802 at the timing of a latch signal (Latch). Only while a strobe signal (Strobe) is set at L level, the output gate unit 803 turns on each output transistor, of the output transistors Q1 to Q64, corresponding to each heat generation element, of the heat generation elements R1 to R64, which receives an H-level output from the latch unit 802. For this reason, the heat generation elements R1 to R64 are driven in correspondence with H-level print data. In this manner, a thermal head apparatus having several hundred to several thousand heat generation elements is arranged such that the number of wiring lines between the thermal head apparatus and an external circuit is considerably small.
However, in this conventional thermal head apparatus, a problem on print quality is posed. That is, low-density printing is performed immediately after printing is started, and, as the printing progresses, high-density printing is performed. This problem is posed because heat for performing printing is stored in the base near the heat generation elements R1 to R64 or the overall thermal head apparatus.
In order to reduce influence of the heat storage, various heat storage correcting methods, i.e., circuits for controlling energy applied to perform printing in accordance with the temperature of a thermal head apparatus are proposed. For example, a method of controlling energy applied to heat generation elements on the basis of the information of a temperature sensor such as a thermistor arranged near heat generation elements to make a printing density uniform is proposed. However, according to this method, the thermal path between each heat generation element and the temperature sensor is long, and the heat response time of the temperature sensor itself is long. Therefore, sufficient heat storage correction cannot be performed.
In addition, a correction method based on printing hysteresis information is also proposed. This method controls energy applied to each heat generation element in accordance with the printing hysteresis of each heat generation element. In this case, since the method is based on print information supplied to each heat generation element itself, energy applied to each heat generation element can be controlled at an accuracy considerably higher than that of the above method using the temperature sensor. When the energy applied to each heat generation element can be controlled by relatively short printing hysteresis information, e.g., when printing such as character printing having a low printing ratio is performed, satisfactory print quality can be obtained.
However, at present, a thermal print scheme is applied to graphic printing. For this reason, in order to obtain good print quality using the above method, long-time hysteresis must be referred to, and print information of the method of arranging the heat generation elements must also be referred to. Therefore, a very large number of integrated circuits are required to practically use the method. In addition, when an image is to be printed, density gradation is required for each color generation dot. For this reason, a conventional print control scheme cannot sufficiently cope with printing of an image.
As a method of solving the above problem, the following method is proposed. That is, a material having an electrical resistance which is largely dependent on temperature is used as a heat generation element, the temperature of the heat generation element is measured on the basis of a change in electrical resistance, and energy applied to the heat generation element is controlled on the basis of the temperature information, thereby preferably performing printing. This method is shown in FIG. 6.
FIG. 6 shows another conventional thermal head apparatus. The same reference numerals as in FIG. 5 denote the same parts in FIG. 6, and a description thereof will be omitted. This thermal head apparatus is constituted by a thermal head base 82 having heat generation elements R1 to R64 formed thereon, a heat generation driving integrated circuit 80, current detecting resistors r1 to r64 each using a single register, a current detection circuit 84, and a control circuit 86 for controlling the elements, the circuits, and the resistors.
The basic difference between the thermal head apparatus in FIG. 6 and the thermal head apparatus shown in FIG. 5 is that the heat generation driving integrated circuit 80 and the like are externally arranged not to be mounted on the thermal head base 82 in FIG. 6. For this reason, a wiring cable having several hundreds to several thousands wiring lines is required between the heat generation driving integrated circuit 80 and the thermal head base 82 and the like. In this manner, the thermal head apparatus shown in FIG. 6 disadvantageously requires a wiring cable having a very large number of wiring lines.
More specifically, the volume of the current detecting resistors r1 to r64 is considerably larger than that of the heat generation elements R1 to R64. For this reason, when the current detecting resistors r1 to r64 are mounted on the thermal head base 82, the thermal head apparatus increases in size. In addition, expensive switching circuits which are equal in number to the heat generation elements R1 to R64, i.e., the current detecting resistors r1 to r64, and used in the current detection circuit 84 are required. When these switching circuits are mounted on the thermal head base 82, not only the thermal head apparatus increases in size, but also the overall thermal head apparatus increases in cost.